Glass compositions and method of making same



Aug. 25, 1959 A. s. PINCUS GLASS COMPOSITIONS AND METHOD OF MAKING SAME Original Filed July 27, 1948 m bnvmwk 2 mwi "Q 1w Aux/s EA/m e 6' PIA/GUS ATTORNEY United States Patent GLASS COMPOSITIONS AND METHOD OF MAKING SAME Alexis G. Pincus, Worcester, Mass, assignor to American OpticalCompany Southbridge, Mass a voluntary association of Massachusetts Original application July 27, 1948, Serial No. 40,963, new Patent No. 2,716,069, dated August 23, 1955. Divided and this application'May 18, 1955, Serial No. 509,355

13Claims. (Cl. 106--47) This invention relates to glass compositions and has particular reference to glasses which have a low index of refraction and alow or controllable optical dispersion and process of making the same.

This application is a division of applicants copending application Serial Number 40,963, which was filed July 27, 1948, and which issued August 23, 1955, as Patent No. 2,716,069.

-ease of fabrication and re'fabrication.

Another object is to provide glasses of the above nature which may be melted and worked at very low temperatures =lying in the region between known commercial glasses and organic plastics and process of making the 'same.

Another object is to provide glasses of the above nature whose indices of refraction may be more positively controlled and over a wider range than known commercial glasses.

Another object is to provide glasses of the above na ture whose dispersive properties may be more positively controlled andover a wider range than known commercial :glasses.

Another object is toprovide glasses of the above nature which include as essential ingredients fluorine and oxygen in varying ratios as anions combined with selected positive elements as cations in controlled related proportions depending upon the resultant characteristics desired.

Another object is to provide glasses of the above nature which include, as an essential ingredient thereof, beryllium fluoride.

Another object is to provide glasses of the above nature which are formed from blends of metaphosphates with beryllium fluoride and other metal fluorides.

Another object is to provide glasses of the above nature having silicofluorides .(fluosilicates) as major constituents.

Another object is to provide glasses of the above nature having aluminum silicofluoride as a major constituent which aluminum silicofluoride can partially or completely replacethe beryllium fluoride.

Another object is to provide glasses of the above na ture having a wider range of relative proportions of network-forming and nonmetwork-forming cations and a wider range of anions than has hitherto been available.

Another object is to provide glasses of the above nature which maybe controlled as to the kinds and relative proportions of anionic constituents present for-desired control of dielectric and optical properties.

Another object is to provide glasses of the above character which utilize as fully as possible available and relatively economical raw materials.

Another object is to provide glasses of the above nature possessing characteristics which make possiblenew fabrieating techniques and new applications. in. coatings, impregnants, cements, enamels, glazes, etc.

Another. object is .to providenovel processes of obtaining all of the above objects and advantages.

Other objects and advantages of the present invention will become apparent from the following description taken in connection with'the accompanying drawing and it will be apparent that :many changes may be made in herein are known :in the art but such known prior art glasses didnot possess sufficient resistance to weathering and ease of fabrication and refabrication to renderthem commercially practical. :Some suggestions were made of the possibilities in beryllium fluoride. glasses from theoretical reasoning'by V. M. Goldschmidt, Skrifter Norske videnskaps-Akad flslo (1926), and very limited research was conducted in the ifield of beryllium fluorideglasses by Dr. George Heyne, Angewandte Chemie, Vol.46, Germany (193-3), but neither inlany way achievedortaught thebroad'scope or findings and resultsobtained by the presentinvention. Heynes contributionztothe art merely included glasses which hestated-werenot stable for practical applications, especially in optics. The beryllium fluoride glasses must, 'where remaining clear fora long time is required, be protected from 'moist air or embedded in-suitablesmaterial. Glasses ofqthe teachings of the present invention do not require this protection and also, .in .certain compositions taught, are more durable than manygglasses-whichare commonly used in opticalgapplications. 1 1

Such known prior art beryllium fluoride glasses embodied compositions utilizing-a-high proportion of relatively .expensiveingredients andxfailed, in general, to provide the art with any definite knowledge as to control of refractive indices and dispersive ,properties and .at most embodied a very narrow range of compositions as compared with the present invention and offered little, if anything, of commercial interest. p

Attempts have also been made to produce lowered refractive index and dispersion "by introducing fluorine into an alkali borosilicate base, free frombivalent oxides, resulting in.the.optical floor-crowns containingup to about 7% fluorine by weight. These glasses did. not providea refractive .index lower than about 1.45. Their dispersion has been tied closely to the index and could not be varied much from V: 67. Compared to the glasses of thepresent invention they are difiicult to melt, give poorquality yields, and are of inferior chemical durability.

Glasses of the present invention open an'entirely new held of research as to optical systems and lens design in generalvand lend themselves toseveral different applications. For examplesof particular usesz such glasses may replace crystals in highly corrected-lens systems; may be used as an intermediateimedium for supporting theelements of different lens systemsnin more positive relation with each other andat the same time replacetthe conventional air spacesiof the elements of such systems with a ing of BeF (beryllium fluoride) or equivalent substituents (PO (the metaphosphate radical) or equivalent substituents, and R (an ingredient or ingredients of the metallic cations group) in such a manner as to obtain glasses having low or variable and controllable indices of refraction, low or variable and controllable optical dispersions, controlled as to stability and resistance to chemical attack and weathering, controllable characteristics to to melting and softening properties, introducing ease of fabrication and possessing practical working and remelting characteristics.

Another method of describing the possible constituents and their relative proportions is by the generalized formula:

m n z l-z) where A represents the hole-filling or non-networkforming cations which may be sodium (Na+), potassium (K+), magnesium (Mg calcium (Ca barium Ba), zinc (Zn lithium (Li+), rubidium (Rb caesium (Cs cadmium (Cd lead (Pb thallium (Tl and (Tl strontium (Sr or the like; B represents the network-forming cations of the type beryllium (Be++), alurninum (Al silicon (Si zirconium (Zr phosphorous (P boron (B sulphur (8 nitrogen (N carbon titanium (Ti or the like. It is understood that certain cations, such as aluminum (Al are able to function as both holefillers and network-formers depending on the chemical balance among the remaining ions; 0 represents the oxygen anion, 0 F represents the fluorine anion, F.

In this generalized formula the sum of the anions present is always taken as unity and the relative proportions of cations present refer to the unit anion. Thus, if fluorine is the only anion it will be given the subscript unity. Where both fluorine and oxygen are present, the fluorine will differ from unity by the proportion x of oxygen present and the fluorine will have the subscription 1-x resulting from subtracting the number represented by x from unity. If other anions such as chlorine, bromine or sulphur are introduced, they will be included in the total within the parenthesis, but the sum of the anions will still be reduced to unity.

The subscrips m and n, respectively, indicate the relative numbers of ions of type A and type B per unit anion.

Examp les I I. Beryllium fluoride (BeF glass would be expressed as Be F.

II. A useful glass composition has been found to be made from the batch:

Percent by weight Beryllium fluoride 40 Cryolite 40 Sodium metaphosphate 20 it being understood that in this formula or wherever referred to herein, the metaphosphate of sodium can be replaced partially or entirely by other known glass forming metaphosphates such as aluminium, potassium, zinc, calcium, beryllium or the like.

Its formula would be expressed as:

by weight of the above batch, the actual weight of the sodium contributed by the cryolite will be equal to 3 22.99 40/209.99, or 13.14%; the actual weight of the aluminum in the batch will be equal to 26.97X40/ 09.96, or 5.14%; and the actual weight of the fluorine contributed by the cryolite will be equal to 6 l9.00 40/209.96; or 21.72%. Therefore, the Na contributed by the cryolite is equal to 13.14/22.99 or 0.571 mole.

Since the sodium metaphosphate constitutes 20% by weight of the batch, the actual weight of the sodium contributed thereby will be equal to 22.99 20/101.98 or 4.51%. Thus, the Na in the batch due to the sodium metaphosphate will be equal to 4.51/22.99 or 0.196 mole. The total Na in the batch due to both the cryolite and the sodium metaphosphate will be 17.65% and accordingly, will be equal to 0.571+0.196 or 0.77 mole.

In like manner, values for the other elements in the batch may be calculated. The aluminum in the batch, as pointed out above, has an actual weight of 5.14% and when the figure is divided by 26.97, it will give 0.19 mole.

The actual weight of fluorine in the cryolite equals 21.72%, as given above, and the actual weight of the fluorine in the beryllium fluoride equals 2X19 40/47.02, or 32.33%. The total fluorine in the batch due to the cryolite and the beryllium fluoride is, therefore, 54.05% and when divided by the atomic weight of fluorine gives 2.84 moles.

The actual weight of the beryllium in the batch is equal to 9.02/47.02 40, or 7.67% and when divided by 9.02 gives 0.85 mole.

The phosphorus in the batch is equal to 30.98/101.98 X20, or 6.08%; and when this figure is divided by 30.98 it gives 0.20 mole. Likewise, the oxygen is equal to 3 16.00 20/101.98, or 9.41%; and when divided by 16.00 gi-ves 0.59 mole.

Applying this information to the general formula A B,,(O,,F it will appear as follows:

Ofl'l( i).85 0.19 0.20) O.59 2.84 With the 19st two values adding up to 3.43. Considering the O+F sum as unity the other values of the formula may be related to unity by dividing each by 3.43. This will give III. Practical substitutions include: For Na-Li, K, Rb, Cs completely; Ba, Sr, Ca, Mg, Zn, Cd, Pb, Bi, Tl, etc. partially. For Be-the minimum is determined by the proportion of fluorine present and Be can be partially replaced by Al, B, Si, Ti, Zr, C, N, P, S, etc.

The above glass and its variations is more fully discussed and is claimed in the parent application Serial No. 40,963 of which this is a division.

By the use of aluminum silicofluoride certain composition regions have been discovered in which the Be can be completely replaced and still obtain glasses having similar characteristics to those set forth above.

0, Cl, Br, I (OH), etc. may be combined with F if desired.

The phosphorus cation P+ has been found to be a particularly useful member of the B group because it brings about a compatibility between oxygen and fluorine anions which greatly promotes glass-forming tendencies and inhibits crystallization or devitrification.

Also within the B group it has been found possible to replace some of the phophorous cations (P with nitrogen (N and carbon (0 with marked improvement in chemical durability of the glasses and unusual optical properties resulting.

By this form of representation the invention as herein claimed is directed particularly to the combining of B network-forming cations with suflicient A nonnetwork forming cations to assure homogeneous vitreous characteristics and with balanced proportions of fluorine and oxygen (and other anions such as chlorine, bromine and sulphur) in such a manner as to obtain true homogeneous glasses having low or variable and controllable indices of refraction, low or variable and controllable optical dispersions, controlled as to stability and resistance to chemical attack and weathering, controllable characteristics as to melting and softening properties, introducing ease of manufacture and possessing practical working and remelting characteristics.

Referring to the drawing:

The figure is a triaxial diagram illustrating glass formations in the field comprising aluminum silicofluoride (Al (SiF beryllium fluoride (BeF and sodium metaphosphate (NaPO and giving therefractive index and nu (V) value of the resulting glasses.

A marking'code has been used to'indicate the melting characteristics of the batches given. This marking code is as follows:

Q (dot within a circle) indicates that a clear glass can be obtained without special precautions or deviations from a normal melting, pouring and cooling cycle.

0+ (a cross beside the circle) indicates that scat: tered crystals were obtained within the clear glass. As is well known these may often be avoided by minor changes in the melting procedures but would introduce more positive control and more difficult procedure.

69 (cross within a circle) indicates that the specimen was still vitreous in nature but contained enough crystals to cause an opalescent, opal or alabaster appearance.

As is well known this difficulty could be avoided and clear glass could be obtained by extreme quenching but again would introduce even greater difficulty in fabrication. The (B therefore indicates the most desirable batches which may be formed with the usual ease of fabrication desired.

(cross alone) indicates that the composition was non-vitreous in its behavior during heating, or cooled to an opaque, fragile mass, predominately crystalline and free from glass and is very undesirable.

The figures above the 6 indicate the index of refraction of that particular batch and the figures immediately below the indicates the nu value (V) and the number immediately below that indicates an arbitrary rating for chemical durability.

In the disclosure set forth above the characteristics listed have been assessed by:

Refractive index and dispersion (nu value) by measurements with an Abbe refractometer on polished faces. The specimens were annealed to remove most of the mechanical strain, but were not fine-annealed or compaoted to and uniform index. It is well known that refractive index values shift with the refinement of annealing approaching maximum value with prolonged annealing at the proper cooling rate. Dispersion valueswould similarly be sensitive to heat treatment. The values given herein, however, are indicative of the order of magnitude of the actual values and are of the type usually relied upon in determining such values to a first approximation.

Chemical durability was rated by appearance of the specimens immediately after polishing and then again after a few months storage in paper envelopes in the laboratory atmosphere.

On the triaxial diagram given:

4 indicates a hygroscopic glass, similar to beryllium fluoride,

3 indicates that the surface formed a white film,

2 indicates that the surface stained,

1 indicates that the surface and the specimen retained its polish and brightness comparable to commercial optical glasses and the glass is considered to possess good durability,

added to the number increases the value, indicating poorer durability.

3 Melting and working procedures and their evaluation will be discussed more in detail later in the specification.

Referring more particularly to said figure it is particularly pointed out that the glasses referred to therein comprise selected portions of aluminum silicofluoride, sodium metaphosphate and beryllium fluoride. The various glasses resulting from the combining of the above three ingredients were measured for index of refrac tion, nu value and durability and the values obtained are noted on the diagram.

Pure "beryllium fluoride (BeF glass has been found to possess extraordinarily low refractive index and optical dispersion. These properties have been found to be as follows:

N =1.27475 N =L27 649 N =1.2 7392 Unfortunately for its usefulness in optical systems this glass is extremely hygroscopic and it is the essence of this invention to teach the obtaining of glasses of the above nature, namely glasses having low refractive index and optical dispersion, with improved chemical durability.

As set forth in the figure other compounds have been gornbined with beryllium fluoride in controlled proportions whereby a homogeneous melt may be obtained and whichcan be cooled to room temperature and re worked without loss of homogeneity.

Also, as shown in the figure, although the index of refraction. increases with the addition of aluminum silicofluoride. and sodium metap-hosphate to the beryllium fluoride, several different indices of refraction and nu values may be obtained by shifting the related proportions of the ingredients whileincreasing the durability of e ass- By referring to the code set forth above, dilferent characteristics of the melts listed in said figure can be easily determined. It is to be understood that the melts set forth therein areonly indicative of the varying characteristics which might be obtained and that by inter polation between spegcific batches set forth, an infinite variety of intermediate batch compositions can be derived with intermediate optical properties. It is believed that the various melts charted are readily determinable and that it is unnecessary to furnish any further disclosure of the related proportions of ingredients of said different melts.

It should be appreciated, however, that the spaced dotted. lines D and E indicate generally on the triaxial diagram the limits within which melts composed of varying amounts of beryllium fluoride, aluminum silicofluoride and sodium metaphosphate give acceptable values for chemical durability (values of l or 2) as indicated thereon.

, Although some of the glasses within the charted area are hygroscopic in nature and less durable than others set forth herein, they, however, may meet the requirements of particular uses, such as for optical elements which may be protected by imbedding them in a resin or by sealing them a lens system, for use as cements, or otheruses.

From the triaxial diagram set forth in the figure, the range of percentages by weight of the various ingredients from which homogeneous structures result are as follows:

Range of, percentagesby weight Sodium metaphosphate (NaPO Approximately 101090 '7 A more practical range from the standpoint of chemical durability of the glasses illustrated in the figure is as follows:

Range of percentages by weight Beryllium fluoride (BeF Approximately to 45 Aluminum silicofluoride (Al (SiF Approximately 1 to 60 Sodium metaphosphate (NaPO "Approximately to 70 Specific examples of compositions which have produced satisfactory results are as follows:

Other specific examples of compositions which have produced good or fair chemical durability and which may befound on the triaxial diagram are:

- A particularly interesting type of substitution is that in which the sodium metaphosphate is replaced not only by other metaphosphates'but by salts supplying other radicals in place of the metaphosphate radical, for example, carbonate-(CO nitrate (NO sulphate (S0 and the like. In certain cases, it has even been found possible to introduce the hydroxyl anion (OH). In all of these cases, sodium has been used as the cation constituents of the salt but has been found to be replaceable by calcium, potassium, zinc and the other A group elements.

In the. case of all of the specific examples set forth above, it is possible to restate the composition in terms of the generalized formula: A B,,(O,,F For illustrational purposes, the glass of batch 12 may be repre sented as follows:

oos( olav ons oos one)o.41( 0.1'z 0.a3)

Although aluminum silicofluoride is here given as a desirable ingredient, it has been found that other silicofluorides may similarly :be incorporated in the glass melts, namely, the silicofluorides of sodium, calcium, zinc and other substitutions for sodium. In terms of the atomic representation, these all come within the A group representing hole-filling cations.

Briefly, again it can be seen that compositions of this type afford the advantage of being able to vary refractive index and nu value independently.

Using any one of these specific compositions as a starting point, a further wide variety of substitutions is possible to modify the optical properties of the resulting glass, to improve the melting and working characteristics, or to employ cheaper and more desirable ingredients. For example, it is desirable to keep the beryllium fluoride content at a minimum because of its high price, difliculty in handling, and volatility. In the example set forth above having a beryllium fluoride content of approximately it has been found possible to partially substitute aluminum silicofluoride for the beryllium fluoride, and also to partially or completely replace sodium metaphosphate by aluminum metaphosphate or other metaphosphates. This is illustrated in the following example:

Approximate Percentages By Weight Batch Batch Batch 11 12 13 Beryllium Fluoride (BeFi) 40 50 Aluminum Silicofluorid Aluminum Metaphosphate Al(PO3)3 Magnesium Metaphosphate Mg(PO3) trols the refractive index level. The higher the fluorine fraction, the lower the refractive index.

. It is to be noted that the. fraction of sodium (Na) varies over a wide range and in certain cases may be omitted entirely. Sodium is quoted in nearly all the examples but glasses have been prepared where the sodium has been replaced by any of the A group enumerated above. Batch 5 illustrates an instance where magnesium (Mg) completely replaces sodium. Among the B group cations it is to be noted that aluminum (Al), phosphorus (P), silicon (Si) and carbon (C) replace beryllium in relatively small fractions but it has been found that even these minor substitutions play an important role in improving the chemical durability and meltability. In atriving at these substitutions it has been found helpful to evaluate the possibilities from the standpoint of the ionic potentials of the element concerned and the cation derived from it. The ionic potential is established by dividing the valence of a cation by its ionic radius. It can be seen that the smaller the radius and the larger the valence the higher is the ionic potential, and high ionic potential has been found to favor a strong structure With good chemical durability, increased hardness and other desirable properties.

Glasses of this unusual type require the use of some relatively novel raw materials for glass making. Beryllium fluoride is available in the form of lumps of high purity berryllium fluoride glass from manufacturers of beryllium chemicals. It has also been obtained in a less pure powdered form. Silicofiuorides (also called fluosilicates) have recently become available in tonnage quantities in the full range of the salts of the A group cations.

As source of a carbon we have found the carbonates advantageous; for example, sodium carbonate (Na CO and calcium carbonate (CaCO The metaphosphates are also commercially available.

The choice of a refractory to hold this type of melt depends on how high the fluorine content is compared to the phosphate. At the highest fluorine contents platinum seemed most desirable and was not attacked directly by the melts, but it wasfound that care had to be taken not to let metallic iron or other metals from alloys come in contact with platinum and the melt as the metal was fluxed by the fluoride-phosphate and in turn attacked the platinum. Fused silica crucibles, graphite and carbon are also practical.

In the high phosphate range ordinary ceramic crucibles (aluminum silicates) have been found to be satisfactory, and in some cases even preferable to platinum.

The choice of melting cycle is also determined by the relative amounts of fluorine and oxygen, that is of fluorides and metaphosphates. Most of these compositions, however, could be melted between 1600 and 1900 F. The higher fluorine glasses fume considerably during this melting stage particularly if the raw materials are damp or contain water of constitution; therefore, the time at this upper melting temperature should be keptas short as possible and for experimental melts of 50 to 250 grams about minutes was ordinarily adequate for complete melting. The lower fluorine glasses are less critical and may be held for longer periods in the melting range. The furnace temperature should then be lowered as rapidly as possible to about 1200 F., depending upon the viscosity and the devitrification tendency of the particular formula, and the melt held at that temperature for about a half an hour or long enough to permit it to calm down and get rid of its fuming and bubbles. Certain of the compositions could be held at temperatures as low as 850 F. without devitrifying and still poured fluidly. Because of the fuming and the extremely fluid nature of these glasses they tend to be quite striated and it is necessary to stir them at the lower temperature. The melts can be poured into graphite or graphitized iron moulds and then annealed. Because of the wide range of compositions taught no specific annealing temperature can be mentioned although 450 F. has been found satisfactory for most of the more desirable formulas. After annealing to release strain and cooling slowly at room temperature, glasses are obtained which can be ground and polished and otherwise handled like conventional glasses. They possess the extraordinary property of being workable at extremely low temperatures, intermediate between the working temperature of normal glasses and organic plastics. The glasses with higher fluorine ratios (fluorine content of 0.50 or higher) have been worked by pressing at temperatures as low as 600 F. glasses with a durability rating of 3 or 4 can be polished by conventional means but it has been found helpful to use a solution of ethylene glycol and water as the liquid suspending the rouge.

These glasses exhibit other unusal properties such as extremely high thermal expansion, low softening temperatures and unusual transmissions in the ultra violet and infra-red regions.

Therefore, from the above teachings, various types of homogeneous structures and specific glass-forming compositions can be produced which permit control of the melting and working properties, control over the chemical durability and control over the optical properties both as to the refractive index level and the relationship between refractive index and dispersion.

This has been accomplished as stated by combining controlled amounts of fluorine and oxygen in varying ratios as anions and selected positive elements as cations in controlled related proportions depending upon the resultant characteristics desired.

The batch formulas, final analysis, percentages, etc. given above are by way of illustration only and should not be limitive of the invention except in so far as they are specifically recited in the appending claims.

From the foregoing, it will be seen that I have produced means and method of a simple and eflicient nature 10 that will produce all of the objects and advantages of the present invention. Having described my invention, 1 claim:

1. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 30% by weight of beryllium fluoride (BeF approximately 5 0% by Weight of aluminum silicofluoride (Al (SiF and approximately 20% by weight of sodium metaphosphate (NaPO said structure having a refractive index of approximately 1.394, and a nu value of approximately 74.

2. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of ap proximately 40% by weight of beryllium fluoride (BeF approximately 10% by weight of aluminum silicofluoride (Al (SiF and approximately 50% by weight of sodium metaphosphate (NaPO said structure having a refractive index of approximately 1.400, and a nu value of approximately 76.

3. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 20% by weight of beryllium fluoride (BeF- approximately 10% by weight of aluminum silicofluoride (Al (SiF and approximately 70% by weight of sodium metaphosphate (NaPO said structure having a refractive index of approximately 1.447, and a nu value of approximately 64.

4. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 20 to 40% by weight of beryllium fluoride (BeF approximately 10 to 70% by weight of aluminum silicofluoride (Al (SiF and approximately 10 to 70% by weight of sodium metaphosphate (NaP O said fused product having a low refractive index ranging between approximately 1.368 and 1.447 and a nu value ranging between approximately 64 and 83.

5. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 20 to 40% by weight of beryllium fluoride (BeF approximately 10 to 70% by weight of aluminum silicofluoride (Al (SiF and from 10 to 70% by weight of a metaphosphate selected from the group consisting of sodium metaphosphate (NaPD aluminum metaphosphate (Al(PO magnesium .metaphosphate (M g(PO beryllium metaphosphate (Be(PO potassium metaposphate (KPO calcium .metaphosphate (Ca(PO zinc metaphosphate (Zn(PO and mixtures thereof, said fused product having a low refractive index ranging between approximately 1.368 and 1.447 and a nu value ranging between approximately 64 and 83..

6. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 40% by weight of beryllium fluoride (BeF approximately 40% by weight of aluminum silico-fluoride (Al (SiF approximately 10% by weight of sodium met-aphosphate (NaPO and approximately 10% by weight of aluminum metaphosphate (Al(PO said fused product having a low refractive index of approximately 1.368 and a nu value of approximately 74.

7. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 20% by weight of beryllium fluoride (BeF approximately 30% by weight of aluminum silico-fluoride (Al (SiF and approximately 5 0% by weight of sodium metaphosphate, said fused product having a low refractive index of 1.434 and a nu value of 72.

8. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 20% by weight of beryllium fluoride (BeF approximately 60% by weight of aluminum silico-fluoride (Al (SiF and approximately 20% by weight of sodium metaphosphate, said fused product having a low refractive index of 1.407 and a nu value of 70.

9. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of ap- 11 proximately 40% by weight of beryllium fluoride (BeF approximately 40% by weight of aluminum silico-fluoride (Al (SiF and approximately 20% by Weight of sodium metaphosphate, said fused product having a low refractive index of 1.370 and a nu value of 83.

10. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 40% by Weight of beryllium fluoride (BeF approximately 30% by weight of aluminum silico-fluoride (Al (SiF and approximately 30% by weight of sodium metaphosphate, said fused product having a low refractive index of 1.384 and a nu value of 73.

11. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately 30% by Weight of beryllium fluoride (BeF approximately 40% by weight of aluminum silico-fluoride (A1 (SiF and approximately 30% by Weight of so- 12 proximately by weight of beryllium fluoride (BeF approximately 70% by weight of aluminum silico-fluoride (Al (SiF and approximately 10% by Weight of sodium metaphosphate, said fused product having a 10W refractive index of 1.394 and a nu value of 64.

13. A homogeneous durable clear transparent optical glass consisting essentially of the fused product of approximately by weight of beryllium fluoride (BeF approximately by weight of aluminum silico-fluoride (Al (SiF and approximately 10% by Weight of sodium metaphosphate, said fused product having a low refractive index of 1.372 and a nu value of 70.

References Cited in the file of this patent UNITED STATES PATENTS Sun et al. Sept, 13, 1949 Pincus Aug. 23, 1955 

4. A HOMOGENEOUS DURABLE CLEAR TRANSPARENT GLASS CONSISTING ESSENTIALLY OF THE FUSED PRODUCT OF APPROXIMTELY 20 TO 40% BY WEIGHT OF BERYLLIUM FLUORIDE (BEF2), APPROXIMATELY 10 TO 70% BY WEIGHT OF ALUMINUM SILICOFLUORIDE (AL2(SIF6)3) AND APPROXIMATELY 10 TO 70% BY WEIGHT OF SODIUM METAPHOSPHATE (NAPO3), SAID FUSED PRODUCT HAVING A LOW REFRACTIVE INDEX RANGING BETWEEN APPROXIMATELY 1.368 AND 1.447 AND A NU VALUE RANGING BETWEEN APPROXIMATLEY 64 AND
 83. 